In FBI slang they are known as "dirty teams" and "clean teams," or as "dark" and "light" agents, or even more cryptically as "fives" and "sixes." The two groups are deployed together when terrorists strike or when top-secret information has gone astray, and they often spend months, even years, working in tandem. Yet they rarely talk to each other.

As the FBI becomes more and more involved in overseas investigations of terrorist threats, using two distinct teams of agents kept apart by an imaginary wall has become a key to separating criminal cases that can be prosecuted in open court from intelligence secrets that must be protected forever.

On one side of the wall are agents privy to top-secret intelligence gleaned from spy satellites or foreign officials whose loyalties and methods may not be fit for public disclosure. On the other side are agents protected from such information so that when they are challenged in court, they need not fear revealing national security secrets or introducing evidence tainted by human rights abuses committed in a faraway jail.

"We find ourselves more and more frequently in situations that require us to protect intelligence assets even as we develop evidence that can be used in a criminal prosecution," Larry R. Parkinson, general counsel of the FBI, said in an interview.

Whether the target is Osama bin Laden, the terrorist chieftain accused in last August's African embassy bombings, or a campaign fund-raiser suspected of working for the Chinese government, this division of labor--once limited to Cold War espionage cases--is becoming a more common feature of federal law enforcement. Following a presidential directive issued last May, the FBI now works intimately with intelligence agencies and the military in foreign counterterrorism operations.

Under the new strategy, criminal prosecutions, guided behind the scenes by intelligence information, have become a favorite means of combating enemies overseas. Senior administration officials said that FBI agents, prepackaged in clean and dirty teams, are regularly deployed abroad in much the same manner as diplomats or aircraft carriers. Bin Laden, for example, has been the target of both cruise missiles and a criminal indictment.

"Gathering intelligence and investigating a criminal case can be just two different ways of going at a similar task, but they each have their own rules, and information that fits in one may not fit into the other," said Victoria Toensing, a Washington attorney who helped develop techniques for international terrorist cases while serving at the Justice Department during the 1980s.

When a case crosses the boundaries between national security and criminal investigation, the FBI sets up one team--the dirty or dark team--drawn from the National Security Division, known bureaucratically as "division five," to handle the intelligence. Another team--the clean or light team--taken from the FBI's Criminal Division, or "division six," builds the case that will be presented in court.

Between the two teams stands an FBI attorney with specialized training who determines what information can be passed from the national security side to the criminal side, screening and packaging the intelligence so that it protects the source but still points the criminal agents in the right direction.

"When you are working a criminal case, the intelligence side can give you valuable information, but it is information, not evidence," said Robert Blitzer, former chief of the FBI's domestic terrorism unit who is now an executive with Science Applications International Corporation. "No matter how good the intelligence, you then have to get witness statements and physical evidence, following all the steps necessary so it can be entered in a court of law. In effect, you have to prove with evidence what you think you know from intelligence."

While the protection of top-secret sources and methods is the most frequent concern, FBI officials said the same basic structure is used to prevent criminal cases from becoming tainted with information acquired by methods that might shock a U.S. court. When the FBI has to deal with foreign police or intelligence agencies that have questionable records on due process or human rights, a national security team takes in the information and, with an attorney, decides what can be properly passed along to the criminal team.

Domestically, the FBI either gathers its own information or draws on other law enforcement agencies well-versed in U.S. criminal practice. But overseas, "almost everything initially comes to us through intelligence channels, either our own agencies or from a foreign government, and so almost everything has to be screened and then reworked by a criminal team," Parkinson said.

That is exactly what took place in the investigation of bin Laden a year ago, after nearly simultaneous bombings shattered the U.S. embassies in Kenya and Tanzania.

On Aug. 7, the day of the bombings, suspected terrorist Mohammed Saddiq Odeh was passing through Pakistan when he was arrested by local airport authorities for carrying a false passport. In the course of nearly a week of interrogation by Pakistani investigators, Odeh admitted that he had helped organize the Kenya bombing, gave key details of the plot and fingered several accomplices, sources said. Most important, he confirmed that bin Laden had directed and financed the bombings from his base in Afghanistan.

That information provided the first break in an investigation that ultimately involved the deployment of more than 500 FBI agents overseas, and it led directly to several other arrests and the seizure of significant evidence in several countries, the sources said. But officially, the FBI never learned of Odeh's confession in Pakistan. Not a word of it appears in court records.

While Odeh was being questioned, Pakistani interrogators allegedly refused to let him eat, drink or sleep for days at a time and threatened action against his pregnant wife, according to one of his attorneys. (The Pakistani government denies these allegations.)

The Pakistanis passed Odeh's revelations to a U.S. intelligence agency, which in turn alerted an FBI intelligence team, sources said. This dirty team, which may not have known the precise origins of the intelligence, worked with an FBI attorney to further filter the information before passing it to a clean team. Meanwhile, the Pakistan government, which had never "officially" advised the U.S. government of Odeh's capture and interrogation, quietly deported him to Kenya, sources said.

In Nairobi, Kenyan authorities advised their U.S. counterparts of Odeh's presence. The FBI's clean team then visited Odeh in jail and, after fully informing him of the rights he enjoyed under the U.S. Constitution, began questioning him. During this interrogation, Odeh admitted to much less than he had in Pakistan. But the clean team, armed with the information provided by the dirty team, knew what questions to ask and where to find corroborating evidence, sources said.

Odeh awaits trial in New York. Due to the efforts of the clean and dirty teams, only information gained under procedures acceptable in a U.S. court has made its way into the indictments naming him, bin Laden and 14 others for their alleged roles in the embassy bombings.

That same basic method has been applied repeatedly during operations overseas, including the ongoing investigation of the 1996 bombing of Khobar Towers in Saudi Arabia, which housed U.S. military personnel; the successful manhunt for Mir Amal Kansi, charged in the 1993 murder of two CIA employees in McLean; and several still-secret missions that preempted planned terrorist attacks and have not yet led to criminal cases.

Some critics argue that the FBI's use of clean and dirty teams encourages the bureau to cut ethical corners when it deploys agents abroad. The federal courts have essentially established the doctrine that U.S. due process and constitutional guarantees only apply after a suspect comes into U.S. custody and that it does not matter much how they got there.

For example, Ramzi Yousef and two co-defendants charged in 1995 in a plot to blow up American airliners sought to suppress statements made to FBI agents after they had been detained by authorities abroad. Rejecting the motions in 1996, U.S. District Court Judge Kevin Thomas Duffy wrote: "The courts of this nation cannot enforce our constitutional guarantees as against foreign governments acting in their own lands."

Such rulings worry some critics. "The whole terrorism scare has given a certain legitimacy to the idea of linking U.S. law enforcement with foreign governments, and when that happens in places like Turkey or Egypt that do not have great records on due process, you have a recipe for losing respect for the rule of law," said Robert E. Precht, who served as a defense attorney in the World Trade Center trial and now directs the public service program at the University of Michigan law school.

To remedy such concerns and avoid pretrial motions to suppress evidence, the FBI's Parkinson said, "we are trying to develop the kind of working relations with foreign police services that allow us to step into a case as early as possible, so we can take custody of evidence and conduct our own interviews of witnesses and suspects. Our goal is to be building our own criminal case from the very start without having to rely on foreign officials."

So far, though, the FBI's most notable difficulty has not been its involvement with foreign governments but internal friction between intelligence teams and their criminal counterparts. For more than a year--and despite intervention by top officials--turf battles have plagued the recent federal investigation into an alleged Chinese plot to buy political influence in the United States, according to an inquiry by the inspector general of the Department of Justice. The report, released last month, found that FBI intelligence operatives were so concerned about protecting their sources of information, they stymied the criminal inquiry.

The FBI is in the process of reorganizing the National Security Division, with an emphasis on resolving the problems highlighted in the inspector general's report.

With counterterrorism as a top strategic priority for the bureau and with a mushrooming number of cases involving intelligence and criminal matters, Parkinson said: "To a certain extent, we are sorting through this as we go along because we constantly have to deal with situations we never encountered before."